Method for improving machinability characteristics of semi-austenitic stainless steels



austenitic stainless steels.

United States Patent fihce 3,082,132 Patented Mar. 19, 1963 Our invention relates to a method of improving the machinability of semi-austenitic stainless steels, and is more specifically directed at reducing the resistance to machining of semi-austenitic, precipitation hardenabl'e stainless steels by a two-cycle heat treatment. This application is a continuation in part of our application Serial No. 772,716, filed November 10, 1958, now abandoned, for Method for' Improving .Machinability Characteristics of Precipitation Hardenable Corrosion Resistant Steels.

In advance design aircraft new materials such as heat treatable stainless steels and highly alloyedheat resistant metals are being employed because of the increasingly severe performance characteristics required. Among the materials being developed for such advanced uses is a class of stainless steels known to the art as semi- These steels range in alloy con tent and in properties between the 400 series martensitic stainless steels which contain a maximum alloy content of about 18 weight percent (in the case of 431 stainless steel), and the 300 series austenitic stainless steels which generally have a minimum total alloywcontent of about 26 percent (for 302 stainless steel). As used here and in the appended claims the term semi-austenitic stainless steel designates stainless steel having a total alloy content of about 18- 26 percent, and falling between the 400 series martensitic and the 300 series austenitic stainless steels in properties. The semi-austenitic steels are low in carbon content and contain relatively substantial quantities of nickel and chromium in addition tomanganese, and molybdenum or aluminum. They resemble' austenitic stainless steels in that they are soft, formable, and austenitic in structure in their annealed condition. On the I other hand, they resemble martensitic st-eels in that after hardening by refrigeration and heat treatment, they have good strength properties and are martensitic in structure.

As examples of the general class, the following table lists the commercial designation and composition of some of these steels:

The machinability of these steels has presenteddiflicult "problems. Intheir as-received austenitic condition from mills, the steels are soft and gummy, which results'in grabbing of the tool.

The action of themachining tool upon the steel causes strain transformation in a virtually instantaneous reaction in the area immediately in advance of the tool. The resulting structure is unnecessarily hard and brittle martensite, and thus the tool encounters at the same time soft and gummy austenite and hard andflbrittle martensite. Such action substantially increases the machining time required and shortens the life of the tool. In addition, the surface finish is sufliciently rough to encourage the development of local stress concentrations.

It is, therefore, a principal object of our invention to provide an improved method of heat treating semiaustenitic, precipitation hardenable stainless steels to substantially improve their machinability.

Another object of our invention is to provide a heat treating method for such stainless steels which decreases the required time for machining a work piece, substantially increases the life of the tool, and improves the machine finish.

Another object is to provide a heat treatment method for semi-austenitic stainless steels which alters their initial soft and gummy nature and prevents strain transformation upon machining.

Other. objects and advantages of our invention will become apparent from the following detailed description.

In accordance with our present invention, the machinability of semia'austenitic, precipitation hardenable stainless steel may be improved by a process which comprises heating the steel to a temperature between about its A2 and A2 temperatures, to cause precipitation oi alloy constituents out of solution. The steel is then cooled to cause phase trans-formation of the initial austenite structure to ferrite. The steel is then again heated to within the Ae -Ae range, and thereafter cooled.

The first heating step causes precipitation from th austenite matrix of alloy constituents such as carbon chromium, etc., which depletes the matrix of the 3110 constituents responsible for causing strain transformatioi hardening during machining, and unstabilizes the austenit structure.

The semi-austenitic stainless steel is heated in the firs heating cycle to a temperature within the Ae Ae range and preferably at or just above the Ae temperaure. Thl Ae temperature is the conventional line on a steel phasl diagram which constitutes the demarcation in an equilib rium condition, between a first region consisting essential ly of ferrite and carbide and a second region consistin; essentially of ferrite, carbide, and austenite. The Ae line is the upper demarcation between this region ant the higher temperature region which is constituted es sentially of austenite and carbide.

The temperatures corresponding to the Ae and Ae lines on the phase diagram will vary with the particula alloy under consideration. The Ae and Ae;-, tempera tures, for example, are lowered with increasing alloy'con tent, and similarly the M and M temperatures are af fected by concentration of carbon and the alloy constitu ents. However, we find that a temperature of abou 1300-l550 F. is generally satisfactory while about 1375 F. is optimum. Heating to higher temperatures is no desirable since the driving force of the precipitation reac tion is diminished, and it may lead to redissolution o the alloy constitutents in the martix, which would resul in undesirable hardening of the steel.

The length of the heating step is not critical and ma satisfactorily vary, depending upon the temperature an the nature of the particular alloy. We find that a heat ing cycle of at least about two hours is generallysatis factory, while about three hours is optimum. The stee is then cooled, such as by air cooling, to a temperatur no higher than the normal room temperature in orde to cause transformation of the initial soft, gummy ans tenite to the ferrite structure which is moderately ban and non-gurnmy. The term ferrite is used herein gs nerically to embrace martensite, bainite and pearlite. Cool ing to room temperature will. generally result in a virtuall complete phase transformation to ferrite, but in order t be absolutely sure of total phase transformation, th steel may be sub-zero cooled, say down to -30 '0 1-00 F.

The first heating cycle improves the machinability o the steel as a result of the precipitation of alloy constituents, and the phase transformation to a less gummy structure. However, the steel is still diflicult to machine due to hardness.

We find that a second heat treatment of the steel at a temperature within the Ae Ae range, as in the first heating step, and also preferably at or just above the Ae markedly increases machinability. The temperature range and time indicated for the first heating step also applies here. The reheating drives still additional alloy constituents out of solution, depleting the matrix. This inhibits any tendency for the alloy constituents to redissolve and further decreases the potentiality for strain transformation resulting from machining. Further, it causes coalescence of the precipitated alloy constituents at the grain boundaries into a fewer number of larger nodules. Because such nodules are very hard, :1 reduction in their number softens the ferrite to a more machinable structure. The material is now of optimum hardness for machinability of semi-austenitic stainless steels, for instance about 33 Rockwell C. Finally, and of considerable importance, many residual stresses resulting from the first cycle are relieved with resulting increases in machinability. Although the reason for the improved machinability resulting from relief of residual stresses in the metal is not known, such relief does in fact improve the machinability.

The steel is then air cooled to a temperature no higher than normal room temperature, or optionally sub-zero cooled, say to -30 to 100 F., to insure total phase transformation. The final aged ferrite structure thus obtained is now ready for machining to final size or for further heat treatment to increase hardness. The following example will illustrate our invention in greater detail.

Example 1 An AM-355 steel casting was heated to 1375 F. and retained at that temperature for a period of three hours after which it was cooled to room temperature. It was then reheated to a temperature of 1425" F. and again retained at that temperature for a period of three hours after which the casting was cooled to room temperature preparatory to machining. In order to provide as objective a basis as possible for a determination of the effectiveness of our heat treating method, comparative machining operations were preformed on two AM-355 steel castings, identical except for the fact that one had been heat treated as above while the second casting was in the as-received mill condition.

Specimens from each of these castings were placed in a No. 2 Cincinnati horizontal mill and machined at varying speeds and depths of cut by a high speed steel disc type mill which was 8" in diameter and 0.6 in width. The same type of cutting tool was used for both castings. Specimens taken from the casting as received from the foundry were rigidly secured to the mill table which was fed past the rotating mill at feed rates which were varied from /2" to 1% per minute for different tests. The table was also set to permit taking depths of cut of A", /2" and 1". The mill was operated at speeds of 40 and 50 rpm. for the different rates of feed. Irrespective of the rate of revolution of the mill, the feed rate, or the depth of cut taken, the cutting blades of the mill were so badly burned and chipped after each 1%" pass of the specimen that the mill had to be completely refurbished.

Specimens taken from the casting heat treated for machinability in accordance with our method were similarly machined with a mill identical to those used in machining the as-received casting specimens, except that only a single revolution rate, feed rate and depth of cut were necessary to demonstrate the superiority of results ensuing as a consequence of our heat treatment. In this instance, the mill was rotated at a 40 rpm. rate and the table set for a /2 depth of cut and a 2 /2" per minute 4 feed rate. Notwithstanding the fact that the 2 /2 per minute feed rate constituted much more extreme machinability test conditions, a single cutting tool successfully machined four passes on each of the two heat treated test 5 specimens before the tool lost its cutting edge.

It will be seen from the foregoing example that the heat treatment of our invention very substantially increases the machinability of semi-austenitic, precipitation hardenable stainless steels. The very substantial degree of improvement of machinability has thus made machining of thees steels commercially practical.

It is to be understood that the foregoing description is by way of illustration only and not by way of limitation, the accompanying claims setting forth the limits of our invention.

We claim:

1. A method of improving the machinability of semiaustenitic, precipitation hardenable stainless steel originally characterized by a soft, gummy structure subject to strain transformation, which comprises heating the steel to a temperature within the Ae Ae range to precipitate alloying constituents from the matrix, cooling the steel to a temperature no higher than about normal room temperature to cause phase transformation, then reheating the steel to a temperature within the Ara -A2 range to .further deplete the matrix of alloying constituents and agglomerate precipitated constituents disposed along grain boundaries, and thereafter cooling the steel to a temperature no higher than about normal room temperature.

2. A method of improving the machinability of semiaustenitic, precipitation hardenable stainless steel initially characterized by a soft, gummy structure subject to unnecessary hardening by strain transformation upon machining, which comprises heating the steel to a temperature of about the Ae temperature, then cooling the steel to a temperature no higher than about the normal room temperature, reheating the steel to a temperature of about the Ae temperature, and thereafter cooling to a temperature no higher than about the normal room temperature.

3. The method of claim 2 wherein the steel is retained at temperature in each of the heating steps for a minimum of about two hours.

4. A method of improving the machinability of semiaustenitic, precipitation hardenable stainless steel initially having a soft and gummy character subject to unnecessary hardening by strain transformation upon machining, which comprises heating the steel to a temperature of about l3001550 F. for a period of at least about two hours to precipitate alloying constituents from solution, cooling the steel to a temperature no higher than about normal room temperature, reheating the steel to a temperature of about 1300-1550" F. for at least about two hours to further deplete the matrix of alloying constituents and agglomerate precipitates disposed along grain boundaries, thereby softening the structure to optimum hardness for machinability, and thereafter cooling the steel to a temperature no higher than about normal room temperature.

5. The method of claim 4 wherein the steel is heated to a temperature of about 1350 F. for about three hours in each heating step.

6. A method of improving the machinability of semiaustenitic, precipitation hardenable stainless steel initially characterized by a soft and gummy structure subject to unnecessary hardness by strain transformation upon machining, said steel having a total alloy content of about 24 weight percent, which comprises heating said steel to a temperature of about 13004550 F. for at least about two hours to cause precipitation of alloying constituents from solution, air cooling the resulting steel to a temperature no higher than about normal room temperature, thereby causing phase transformation to ferrite,

reheating the resulting hardened steel to a temperature of about 13001550 F. for a period of at least about two hours in order to further deplete the resulting austenite matrix of alloying constituents and agglomerate the precipitate bodies disposed along grain boundaries, thereby softening the structure to optimum hardness for maehinability, and thereafter air cooling the resulting steel to a temperature no higher than about normal room temperature.

7. The method of claim 6 wherein each heating period is for about three hours, and wherein at least one of the two cooling steps is to a sub-zero temperature.

8. A method of improving the machinability of semiaustenitic, precipitation hardenable stainless steel having a soft and gummy structure subject to strain transformation .upon machining and having approximately the following alloy constituents, by weight percent: C, 0.12; Cr, 15.5; Ni, 4.50; Mu, 1.00; Si, 0.40; and Mo, 2.75, which comprises heating said steel to a temperature of about 1375 F. for about three hours, air cooling the steel to about normal room temperature, then reheating the steel to a temperature of about 1425 F. for about three hours, and again air cooling the resulting steel to about normal room temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,380,821 Breeler et a1 July 31, 1945 2,506,558 Goller May 2, 1950 2,797,993 Tanczyn July 2, 1957 

1. A METHOD OF IMPROVING THE MACHINABILITY OF SEMIAUSTENITIC, PRECIPITATION HARDENABLE STAINLESS STEEL ORIGINALLY CHARACTERIZED BY A SOFT, GUMMY STRUCTURE SUBJECT TO STRAIN TRANSFORMATION, WHICH COMPRISES HEATING THE STEEL TO A TEMPERATURE WITHIN THE AE1-AE3 RANGE TO PRECIPITATE ALLOYING CONSTITUENTS FROM THE MATRIX, COOLING THE STEEL TO A TEMPERATURE NO HIGHER THAN ABOUT NORMAL ROOM TEMPERATURE TO CAUSE PHASE TRANSFORMATION, THEN REHEATING THE STEEL TO A TEMPERATURE WITHIN THE AE1-AE3 RANGE TO FURTHER DEPLETE THE MATRIX OF ALLOYING CONSTITUENTS AND AGGLOMERATE PRECIPITATED CONSTITUENTS DISPOSED ALONG GRAIN BOUNDARIES, AND THEREAFTER COOLING THE STEEL TO A TEMPERATURE NO HIGHER THAN ABOUT NORMAL ROOM TEMPERATURE. 